(1) Field of the Invention
The present invention relates to an anti-corrosive coating composition providing a tough coating excellent in corrosion resistance on an iron or steel substrate, and also to a process for forming such a coating by using this anti-corrosive coating composition.
(2) Description of the Prior Art
Resin linings for preventing corrosion and rusting of metals have been used for various pollution-preventing apparatuses and chemical apparatuses and also for tanks, oil tanks of ships and ship bottoms.
From the viewpoint of the normal temperature curing property in situ operation adaptability and the manufacturing cost, unsaturated polyester resins have heretofore been used for formation of these resin linings. As the lining process, there has ordinarily been adopted the so-called FRP lining process in which an organic peroxide is incorporated in advance into a resin, a sheet-like substrate composed of glass fibers is applied to a material to be lined, the sheet-like substrate is impregnated with the resin by using a felt roll or the like and the resin is cured while simultaneously effecting defoaming.
In addition to this FRP lining process, the so-called flake lining process, which comprises coating a composition comprising a resin and a very thin flake composed of a vitreous substance onto a material to be coated by using a trowel has recently attracted attention in the art and has already been put into practice.
As the composition that is used for practising this process, there are known, for example, a protective or decorative coating composition comprising fine glass slices incorporated in an organic resin binder vehicle (see Japanese Patent Publication No. 25368/76) and a lining material formed by filling a lining resin with a glass flake as an anti-corrosive substances and a glass fiber as a reinforcer (see Japanese Patent Application Laid-Open No. 30855/77). Furthermore, there is known a plastic material, paint or insulating paper filled with glass scale which has been rendered hydrophobic or provided with a leafing property by surface treatment with an appropriate material (see Japanese Patent Publication No. 16821/72).
In view of properties of the resulting coatings or films, all of these known compositions are applied to a very great thickness exceeding 2 mm so that the coating operation becomes very expensive. Accordingly, these compositions have only rarely been applied to ordinary steel structures but are used only for very limited purposes such as mentioned above.
If a flake lining now available is used in the form of a film having a thickness of less than 2 mm with a view to reducing the cost, it is impossible to impart the intended properties to the resulting coating film, and hence, such film cannot be put into practical use.
In the above-mentioned composition including glass flakes, in order to effectively exploit the characteristics of glass flakes and control the permeation of corrosive ions by the glass flakes, how to arrange the glass flakes in parallel with the surface of a substrate is a very important problem. In the conventional flake lining process using a coating composition including glass flakes, parallel arrangement of the glass flakes is promoted by levelling the coated surface with a trowel or roller. However, the running cost is increased if such trowel or roller is used, and this process cannot be applied to a structure having a complicated configuration.
As means for eliminating these difficulties involved in the coating operation, there has been proposed a process using a glass flake-containing composition that can be sprayed. For example, there is known a process for forming a polyester laminate, which comprises spraying the surface of a metal with a slow gelling composition prepared by (1) mixing from 30 to 95% by weight, based on the composition, of a polyester resin, from 15 to 50% by weight, based on the composition, of a cross-linking agent and up to 50% by weight, based on the composition, of glass, (2) adding to the mixture prepared in step (1) a product obtained by reacting from 0.1 to 0.5% by weight, based on the composition, of pentanedione with from 0.5 to 1.5% by weight, based on the composition, of a metallic promotor and (3) adding about 1% by weight, based on the composition, of a catalyst capable of generating free radicals to the mixture of step (2) (see U.S. Pat. No. 3,653,954). However, even if spray coating is carried out by using the above-mentioned composition, glass flakes are not arranged in parallel with the surface of a substrate and deviation of the parallel arrangement according to the coating direction (horizontal plane and vertical plane) is large. Accordingly, manifestation of the effect of glass flakes for controlling permeation of corrosive ions can hardly be expected.
As is seen from the foregoing illustration, a coating process or coating composition ensuring substantially parallel arrangement of glass flakes to the surface of a substrate has not been known. In particular, when the coating composition is applied in the form of a thin film, rusting is readily caused from pinholes, resulting in serious defects. More specifically, rust formed on pinholes grows rapidly, and rusting is not limited in the pinhole portion but propagates to the interface between the film and the iron or steel substrate, with the result that such defects as blistering and peeling appear.